Paper machine couching box with a multiperforate surface

ABSTRACT

A couching box for facilitating separation of a paper web from the forming wire includes means adapted to act momentarily on the moving wire and web to withdraw air and water from a supported area of the wire and web between the couch roll and the turning roll, and to release the web quickly and evenly from such action just prior to its separating from the web. This facilitates separation of the web, reduces stress on the paper, minimizes web breaks, and increases production efficiency. The couching box has a hard plane multiperforate surface which is separated from an imperforate surface by an intersecting groove. Vacuum for air and water removal from the web is effective through the multiperforate surface and in the groove.

S p 11, 1973 W.IE. HAWKINGS 3,758,380

PAPER MACHINE COUCHING BOX WITH A MULTIPERFORATE SURFACE Filed July 6, 1971 2 Sheets-Sheet 1 INVENTOR 75 William E. HAWKINGS Sept. 11, 1973 w. E. HAWKINGS PAPER MACHINE COUCHING BOX WITH A MULTIPERFORATE SURFACE Filed July 6, 1971 2 Sheets-Sheet 2 FIG. 5

INVENTOR William E. HAWKINGS ATTORNEY f United States Patent 3,758,380 PAPER MACHINE COUCHING BOX WITH A MULTIPERFORATE SURFACE William E. Hawkings, Alma, Quebec, Canada, assignor to The Price Company, Limited, Quebec, Canada Filed July 6, 1971, Ser. No. 159,599 Int. Cl. D2113 2/00 U.S. Cl. 162-306 5 Claims ABSTRACT OF THE DISCLOSURE A couching box for facilitating separation of a paper web from the forming wire includes means adapted to act momentarily on the moving wire and web to withdraw air and water from a supported area of the wire and web between the couch roll and the turning roll, and to release the web quickly and evenly from such action just prior to its separating from the web. This facilitates separation of the web, reduces stress on the paper, minimizes web breaks, and increases production efficiency. The couching box has a hard plane multiperforate surface which is separated from an imperforate surface by an intersecting groove. Vacuum for air and water removal from the web is effective through the multiperforate surface and in the groove.

BACKGROUND OF THE INVENTION This invention relates to the papermaking process.

(1) Field of the invention More particularly it relates to the part of the process at which the Wet sheet of paper is separated from the drainage element against which it is first formed from a dilute suspension of fibres, for transfer to a pressing section where it is further dewatered.

(2) Description of prior art The conventional papermaking process comprises a series of steps of which the following are typical. A mixture of pulp fibres, often With other materials added, is dispersed in water to a consistency of about 0.2% to about 1% and is flowed on to a travelling open weave belt (called a wire) at a rate commensurable with the speed of the wire and the desired basis weight of the paper to be made. Water drains through the travelling wire under the effect of gravity supplemented by the action of devices under the wire which serve both to support the wire and promote water removal through it. These devices may be rotating rolls driven by contact with the wire, or stationary devices such as foils, or boxes having perforations by which vacua maintained in the boxes aspirate water through the wire. By such means, water is progressively removed from the fiber mass and the watery suspension is converted into a wet, weak paper web. Usually, further water is removed by passing the wire over perforated suction boxes where higher vacua suck more water through the wire and further dry, consolidate and strengthen the web. The final action on the web in contact with the wire belt is usually applied as the wire passes over a vacuum couch roll in which is contained a vacuum chamber acting through perforations in the couch roll to suck further water from the paper web.

Following the couch roll, the paper web is stripped from the wire and transferred, usually to a fabric felt which carries it to devices in which further water is removed by pressing between heavy rolls. Following these, the paper is generally dried by sequential contact with a number of rotating steam heated cylinders and may be further processed by calendering, coating or other treatment appropriate to its destined use.

Patented Sept. 11, 1973 In recent modifications of the paper making process, initial dewatering of the original, dilute pulp suspension may take place with the pulp suspension confined between two Wires. In such cases the initial dewatering may be effected by a mechanical pressure exerted by the wires upon the stock between them. Further dewatering in such cases is effected by perforated vacuum boxes in contact with one or both of the wires, or by perforated rolls containing vacuum boxes as before described. In such twowire machines one wire is eventually removed from contact with the wet paper web and subsequently the web is removed from the second wire and continues to a pressing section as described.

The paper web, when transferred from the wire to the press section is normally composed of to water, is very fragile, and is often travelling at high speed. It is therefore very subject to breaking at this point with consequent loss of paper production and profits.

SUMMARY OF THE INVENTION It is an aim of the present invention to provide a method to overcome the problems discussed and to facilitate removal of the paper web from the wire to the press section, as will become evident from the following.

The invention is applied to a paper machinet wet end which has in series, a head box, a continuous loop foraminous web having a face positioned to receive, from the head box, paper furnish in the form of a dilute fiber suspension, means of removing water from the suspension, a vacuum couch roll within the loop for further water removal, and a turning roll within the loop. The wet web is separated from the foraminous web and has last contact with it along a line located between the couch roll and the turning roll. In accordance with the invention the web is subjected to suction to remove air and water from it at a zone between the couch roll andthe turning roll while at the same time the foraminous web is positively supported in that zone. This is preferably accomplished by apparatus according to the invention which includes a hard, plane multiperforate surface extending across the width of the foraminous web and in sliding contact with it at a position between the couch roll and the turning roll. Means is proru'ded to apply a vacuum to the underside of the multiperforate surface so as to remove air and water from the web. Support means is provided for retaining the multiperforate surface parallel to and an contact with the foraminous web. Preferably, the multiperforate surface has arr imperforate land area adjacent to its trailing edge, the edge with which the moving foraminous web has last contact and this land area is provided with a groove across it, disposed so that its trailing edge intersects the surface along a straight line contiguous to the land area. There is provided an operative connection between the groove and the plurality of holes closest to the land area, so that vacuum applied to said holes will be effectively applied to the groove and the paper web overlying it.

The conditioning of the web by withdrawal of air and water from it in the zone just prior to its leaving the foraminous surface, achieves the following results. Adhesion is reduced between the web and foraminous surface, thus facilitating their separation. By decreasing its water content the sheet is strengthened immediately prior to its removal from the wire. The line of separation of the sheet and the wire is more nearly a perfect straight line than is otherwise the case. This reduces stresses in the paper. A dryer sheet is presented to the pressing operation so that this operation can be more effectively performed. These results make possible the overall objectives of reducing the number of web breaks and increasing production efliciency.

3 BRIEF DESCRIPTION OF DRAWINGS The invention will be explained in more detail by reference to the accompanying drawings, which illustrate typical machinery and show preferred embodiments of the present invention, and, in which- FIG. 1 is a diagrammatic cross-sectional view of a type of machine, at the point of transfer of a paper web from the forming wire to the press section, in the wet end of a paper machine, which in this case embodies what is colloquially known as a poor mans pick up.

FIG. 2 is a fragmentary diagrammatic cross-sectional view of a paper machine embodying one form of the invention.

FIG. 3 is a fragmentary front elevation of the wire part of a Fourdrinier machine, equipped according to the invention.

FIG. 4 is an enlarged transverse vertical cross-section through the couching box shown in FIG. 3.

FIG. 5 is a fragmentary plan view showing the top surface of the counching box of FIGS. 3 and 4.

FIG. 6 is a fragmentary diagrammatic cross-sectional view of the relevant portion of a couch vacuum transfer for a Fourdrinier paper machine, equipped according to the invention.

FIG. 7 is a fragmentary diagrammatic side view similar to that of FIG. 2 showing an alternate form of vacuum box 31'.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS The physical conformations of paper machines, at the point of transfer of the paper web from the forming wire to the press section are diverse. By way of illustration of a commonly used conventional arrangement there is shown in one conventional conformation, embodying what is colloquially known as a poor mans pick up. In FIG. 1, 21 indicates the forming wire bearing the wet paper web 22 and travelling in the direction indicated by the arrow 29. The wire 21 partially wraps the couch 23, the surface of which is perforated by numerous small holes. Fixed within the rotating couch roll 23 is a stationary vacuum box 24 by which vacuum is applied through the holes of the couch roll to suck water from the paper and through the wire over the open area of the vacuum box. The wire 21 passes around the forward drive, or turning roll 26 returning in a continuous closed loop to have more fibre placed on it for drainage. Rolls 23 and 26 are usually motor driven to provide the power input moving the wire around its closed loop.

The paper sheet is removed from the wire adjacent to the draw roll 25 and is transferred to a fabric press felt 28 for transportation through a first pressing section. Roll 25 serves to define and control the path of the paper between the Wire 21 and the press felt 28.

Paper breaks often occur in the span between the wire 21 and the press felt 28 due to the stresses which must be applied to controllably transport the unsupported paper web across this gap. The stresses in the web arise from the necessary condition that the press felt 28 be travelling at a higher speed than the wire 21. This speed difference may be in the range of 1% to 6% of the wire speed and stretches the paper in the same measurein order to develop in the paper the tensile force required to resist the external forces acting on it. These forces are: (a) that required to peel the paper off the wire and, (b) that required to resist the centrifugal force associated with the curved path of travel of the sheet. An additional small tension in the paper is required to assure that it is held in a path such that it maintains contact with the draw roll 25. The tension necessarily developed in the wet web to achieve these ends is usually a large portion of the full, normal, tensile strength of the wet paper. Minor changes in the speed relationship of wire and press felt, flaws in the sheet, local spots of stronger adhesion of sheet to wire, and other factors, may stress the sheet to the breaking point and interrupt production.

FIG. 2 shows part of a paper machine wet end such as is shown in FIG. 1 (like numbers being used to designate like parts in two figures) but adapted to use the present invention. Besides the elements present in FIG. 1, there is shown a schematically illustrative modification of the invention.

Additional to the parts described with reference to FIG. 1, in FIG. 2 is located a suction box 31 having a schematically indicated support means 32 by which a long, narrow smooth, perforated surface of the box 31 is held in contact with the inner side of the wire loop 21 at right angles to the direction of travel of the wire at the location between the couch roll 23 and the turning roll 26. The box 31, the draw roll 25, the felt roll 27 and its felt 28 are so relatively positioned that the course of the paper is such that it is peeled from the wire at a point closely adjacent to that edge of the surface of the suction, or couching box 31 in contact with the wire which has last contact with the moving wire.

The couching box 31 is rigidly supported, at each of its ends, from the paper machine frame or from the machine base on each side of the machine. Support means, not shown, include adjustments by which the perforated contact face of the box can be precisely located paralled to, and in light contact with, the inner surface of the wire.

The couching box 31 is shown in FIGS. 2 and 6 as having a generally rectangular cross section. It might also have other shapes, for instance, it may be a box 31 of FIG. 7 of trapezoidal cross-section such that one of its acute angled corners may be fitted into the space between the separating wire and couch roll surface to provide the couching effect at a point closer to the wirecouch roll separation point should this be desirable in a specific couch-topress transfer geometry.

A vacumm is maintained within the suction box 31 by a vacuum pump, or other means, appropriately connected to the box. This vacuum, by way of the multiplicity of small holes in that surface of the box contacting the wire, draws water from the paper sheet into the interstices of the wire 21 and, in part, into the box 31 from which it is removed. In addition to strengthening the sheet by decreasing its water content, use of the box 31 has the equally important effect of reducing the adhesion of the paper to the wire, thus reducing the forces required to separate wire and paper and permitting a reduced tension in the paper and a reduction in likelihood of its breaking.

The vacuum in the couching box 31 creates a pressure imbalance across the wire-paper composite passing over it. Atmospheric pressure on the paper compresses the paper structure and expresses water from it into and through the wire mesh and, in some part, into the couching box for removal. Immediately after a cross machine element of wire and paper leave the vacuum effect of the box, the pressure imbalance is removed. The fibre mat, having been compressed, and being resilient, then increases in thickness. In so doing the paper aspirates some of the Water held in the mesh of the wire back into the sheet structure. As a result of this aspiration there is a momentary condition in which a substantial amount of water exists at the wire-paper interface and this water facilitates separation of the paper from the wire. Eventually, though in a very short time, this Water from the wire mesh is resorbed into the paper, and removal of the paper from the wire becomes more difficult again.

When the paper sheet being made is travelling at 1800 feet per minute, the distance after cut off of the couching box vacuum through which reduced sheet-to-wire adhesion lasts is usually less than 2 corresponding to a time lapse of 0.005 second, or less. The actual distance and time through which the beneficial eifect of the couching box is exerted will depend, in specific cases, upon the speed of the paper machine and the nature of the fibres used. In all cases, though, it is preferable to separate the sheet from the wire immediately after cut-off of the vacuum.

By the use of the apparatus of the invention, it has been found that the dry fibre content of the paper leaving the forming wire may be increased by about 3 percentage units, or about from 19 to 22% dry content in the case of newsprint. This reduction in water content substantially strengthens the wet web and reduces the frequency of breaks at the transfer point. Additionally, due to easier separation of paper from the wire and the higher strength consequent to higher dryness, the strain required to be maintained on the sheet across the gap between the wire and the press felt is reduced by about 10% which further reduces the likelihood of web breaks.

The apparatus of the invention is explained in more detail with reference to FIGS. 3, 4 and 5. FIG. 3 is a front view of the wire part of a Fourdrinier-machine equipped according to the invention. At 21 is shown the Fourdrinier wire cut back to show the perforated couch roll 23 and its interior suction box 24. A paper web 22 is shown on part of the wire. The couching box is shown as 31. The wire 21 travels from the couch roll 23, over the couching box 31 in parallel contact with it and then around the forward driving roll 26. As before the direction of wire and paper travel is indicated by the arrow 29. The dotted line 33 indicates the approximate location of the line along which the paper is separated from the wire.

FIG. 4 is a cross sectional view of the couching box 31 of FIGS. 2 and 3 and enlarged as compared with the latter. It consists of suitable airtight case 34 having an opening 35 leading to a conduit adapted for connection to a vacuum pump or other aspirator. Sealably seated in an opening in the box 34 is a multiperformate element 36 which in this case is a date plate containing holes 37 communicating with the interior of the box and sealabbly nesting in a recess 34a provided on the upper rim of the box.

The surface of the element 36 may be made of any of a number of materials, of the type used for suction box covers in the paper industry. A preferred material is silicon carbide, or material of like hardness and low coetficient of friction. These properties are desirable because the relative dryness of the paper at this point and the relatively high vacuum used in the couching box 31 would otherwise lead to rapid wire wear and high power requirement for pulling the wire whose surface it engages frictionally over the box.

FIG. shows a fragmentary plan view of part of th couch box cover 36. Circular holes 37 are preferred as the perforations in the cover 36. These holes may vary in diameter and spacing, but it has been found that about inch diameter at about /8 inch spacing is a suitable combination. Slots, rather than circular holes may be used. These may be in the patterns known to be used in suction flat boxes on paper machines, but are preferably made with narrower slots for use with the invention. Narrower slots are desirable because the vacua used in the couching box are usually higher than those used for flat boxes: wide slots may result in the wire dimpling into the slots causing excessive wire wear. The size, numbers and distribution of holes should be effective to apply vacuum to the paper sheet, while allowing enough imperforate surface to provide a bearing surface effective to support the forming wire 21 running in contact with it. An effective range of perforated to total surface area of the perforated support surfaces is from about 20% to about 30%. An effective size range from the openings is from about 7 inch to about inch diameter.

In one adaptation of the invention, a slot or groove 38 is cut into the surface of the element 36 near that edge of the element at which the wire leaves the element. This slot 38 has a straight edge 39 on the side closest to the edge of the element 36 and its other edge 40 is scalloped as it intersects the last row of the holes through element 36. The size of the slot is not critical, but can appropriately be about 7 inch wide and of like depth with its near edge about 1 inch from the downstream edge of element 36. An effective slot width is within the range from about 7 inch to about inch. The slot 38 intersects the last row of holes of the couching box cover 36 and is linked by them to the interior of the 'box 36. The vacuum in the box 36 is thereby operative along the slot 38. The width of the perforated portion of the surface of the couching box may suitably be 4 to 10 inches in the direction of wire travel, greater widths being used for higher machine speeds. The straight edge 39 of the slot 38 forms the leading edge of a flat land 45 lying in the plane of the surface of the element 36.

The function of the groove 38 is to assure a sharp cut off of vacuum under the wide and web along a straight line. Stripping of the sheet from the wire occurs just past the downstream edge of the slot 38 along a line which is straight due to the straight line of release of vacuum provided by the existance of the slot. In the absence of this slot vacuum release would occur sooner in the gaps between the last downstream row of holes than it does at the holes themselves. This would result in an irregular line of separation of paper from wire and develop a pattern of stress areas across the paper; as a result, the hazard of web breaks would be somewhat greater than when vacuum is terminated along a straight line such as achieved by providing the straight-edged slot 38.

The slot 38 has been shown and described as intersecting the holes 37 of the last row as a means of connecting the slot to the interior of the box 34 so that the vacuum therein may be operative in the slot. This may be replaced by other means of providing for the appilcation of the vacuum in slot 38. One such way of doing this would provide a slot adjacent to, but not intersecting the ultimate row of holes, with the slot itself having holes through its bottom opening into the interior of the box 34, so as to apply vacuum to the slot.

In the use of the couching box of the invention it is desirable to have a vacuum within it of 2" to 24" of mercury. The higher the vacuum, the drier and stronger will be the paper sheet as removed from the wire and the fewer will be the web breaks. Higher vacua press the wire more tightly against the couching box surface thereby increasing frictional drag which results in an increase of paper machine drive load and rate of wire wear. Optimum operating vacuum will, therefore, depend upon the specific circumstances of each application of the invention. A preferred range of vacuum is from about 10 to about 15 inches of mercury.

The initial transfer of the paper web from the Fourdrinier wire is usually eifected as follows. A fine, high pressure water jet is impinged on the wet sheet before the couch roll 23 to cut the wet paper web continuously along its length about 2 to 4 inches in from one edge. After the main web, and the narrow strip so separated from it, have gone past the couch roll suction box 24, an air jet located under the wire is turned on to blow only the narrow strip from the wire. Usually the appropriate end of the draw roll 25 is raised so that paper strip may be blown onto, and carried into the press section by the press felt 28. When the strip has been thus transferred from wire to felt, the high pressure water jet is moved across the paper machines width to broaden the originally narrow strip until it embraces the full width of the paper web. When the couching box of the present invention is used, the same means of transferring the sheet from couch to press may be used. In this case, the air jet used to blow the initial narrow strip from the wire should be located immediately following the couching box. In an alternative arrangement the air jet may be located between the couch roll and the couching box.

In this arrangement, it is necessary to cut off the vacuum in the couching box to prevent the narrow paper strip from being held to the wire.

The vacuum may, for the purpose of getting the paper from wire to press felt, be cut off the whole length of the couching box, or the couching box may be provided with a small, partitioned section at one end from which vacuum may be cut off during this procedure. In other variants, the partitioned-off section may contain the air nozzles which provide the air jets for blowing the paper strip from the wire, or they may be located just after the couching box.

While the invention has, up to here, been described with particular reference to the Fourdrinier paper machine with a poor mans pick up, it is useful with other types of papermaking machines in which separation of the paper from the wire occurs some distance after the paper and wire have passed the last prior unit applying suction through the wire to the paper.

One such application is shown in FIG. 6. This is a diagrammatic representation of the pertinent portion of a couch vacuum transfer Foundrinier paper machine. This configuration differs from that of FIGS. 1 and 2 in that the paper is transferred from the wire to a pick up felt 42 which passes around a perforated suction pickup roll 43 within which is located a vacuum box 44. The pick up roll 43 is mounted on the free end of a pivoted arm by which it can be brought into contact with the paper 22 on the wire 21. Vacuum within the box 44 acting through the perforations of the shell of roll 43 and the felt 42 draw the paper to, and hold it on the felt for passage to the press which follows.

In operating position the roll 43 and the felt 42 slightly indent the wire 21 so that rather than running in a single tangent from the couch roll 23 to the forward drive roll 26, the wire follows a tangent from the couch 23 to the vacuum transfer roll 43, slightly wraps the latter roll and then follows a second tangent, at a slight angle to the first to the roll 26. This indentation of the wire assures good contact between the surface of the paper 22 and the pick up felt 42 for effective paper transfer.

In the application of the couching box of the invention to this type of vacuum transfer, the perforated surface of the couching box is so mounted as to lie in the plane of the under side of the wire along the wires tangent between the rolls 23 and 43. For this application the land 45 may suitably be made about 2" wide, increasing its width downstream of the slot 38 to pro vide a bearing surface to support the pressure applied by the roll 43 upon the wire 21. The position of the suction box 44 in the vacuum transfer roll 43 will be such that its upstream seal will usually be over this portion 45 of the couching box cover, thus assuring that the vacuum effect of box 31 has ended before that of box 44 becomes effective.

Another modification is desirable when the couching box of the invention is used in the arrangement just described. It is desirable that the couching box 31 be resillently supported, as indicated by the leaf spring 41, to prevent possible development of high pressures between the roll 43 and the couching box surface which might cause damage to the sheet or wire. Minor adaptions of mode of mounting permit advantageous use of a couchmg box, according to the invention, on other types of paper machines, such as. the twin wire machines known as the Bel Baie, Vertiforma and Papriformer, where there is an adequate span of wire between the couch roll and the point of wire-paper separation.

What is claimed is:

1. In a paper machine wet end having, in series a head box, a continuous loop foraminous web having a surface positioned to receive a dilute fibre suspension from said head box, means for removing Water through said web to form from said suspension a wet paper sheet, a vacuum couch roll within said loop for further water removal, a turning roll within said loop and means for the separation of said wet paper sheet from said foraminous web along a transverse line located between said couch roll and said turning roll, the improvement comprising a suction box located transversely across the inside of the loop of the foraminous web in operative relation therewith adjacent the transverse line of separation of the wet paper sheet from the foraminous web and support means for said suction box, said suction box being particularly characterized by having a hard plane multiperforate surface in contact with the foraminous web, a box sealably related to and supporting said multiperforate surface the perforations of which communicate with the interior of said box, a connection to the interior of said box for removal of air and water therefrom by means of an externally supplied vacuum, an imperforate land area adjacent that edge of said multiperforate surface at which the moving foraminous web has last contact with said multiperforate surface, a groove extending across said multiperforate surface, the groove being so disposed that one of its edges intersects said surface along a straight line contiguous with said imperforate land area, said groove having operative connection with the interior of said box whereby vacuum in the box will be effective in said groove.

2. The improvement defined in claim 1 in which the multiperforate surface contains a plurality of holes, and has an imperforate land area adjacent that edge of said multiperforate surface at which the moving foraminous web has last contact with said multiperforate surface being provided with a groove across it disposed so that one edge of the groove intersects said surface along a straight line contiguous to said land area, said groove having operative connection with those of said plurality of holes closest to said land area whereby vacuum within said box will be effective in said groove.

3. The improvement defined in claim 1, wherein said support means provides a resilient mounting for the box, said multiperforate surface having an imperforate land area wider than 1 inch adjacent the edge of said surface having last contact with the foraminous web, and a vacuum transfer means is so positioned as to bear on the wet paper sheet lying over the supporting foraminous web where said web lies over the imperforate land area.

4. The improvement as defined in claim 3, in which the perforated area of the multiperforate surface and the said imperforate area are separated by a groove, said groove having a straight edge adjacent the impeforate land and having connection with the interior of said box whereby the vacuum under the multiperforate surface is effective in said groove.

5. The improvement as defined in claim 3, in which the perforated area of the multiperforate surface and the said imperforate area are separated by a groove, said groove having a straight edge adjacent the imperforate land and having connection with some of the perforations whereby the vacuum under the multiperforate surface is effective in said groove.

References Cited UNITED STATES PATENTS 2,990,013 7/1961 Rance et al 162-307 3,067,816 12/1962 Gould 162-374 S. LEON BASHORE, Primary Examiner R. V. FISHER, Assistant Examiner US. Cl. X.R. 162-374 

